Increased and randomized output sampling to reduce positioning noise in a data storage system

ABSTRACT

A method and system for reducing positioning noise in a data storage system are provided. An access device is positioned over a data storage medium using a stream of bursts stored on the medium. The bursts are sensed at a frequency determined by the rate at which the medium is moved relative to a sensing device. Output positioning values are provided to position the access device at a frequency higher than the frequency at which the positioning bursts are sensed, and/or at random times. The random times may be calculated as random advances or delays from time points occurring at a fixed frequency.

RELATED APPLICATION INFORMATION

This application is a divisional of U.S. patent application Ser. No.08/692,164, entitled “Increased and Randomized Output Sampling to ReducePositioning Noise in a Data Storage System,” filed Aug. 5, 1996 now U.S.Pat. No. 6,181,507, which Application claims the benefit of U.S.Provisional Application No. 60/007,989, entitled “RandomizedOversampling Servo Algorithm for Low Seek Acoustics,” filed Dec. 5,1995. The Provisional Application is hereby incorporated by referenceherein in its entirety.

TECHNICAL FIELD

This invention relates to data storage systems. More particularly, thisinvention relates to a method and system for reducing the noiseencountered when providing positioning values to an access device toposition the access device over a data storage medium.

BACKGROUND OF THE INVENTION

Conventional sector servo direct access storage devices (“DASDs”) have afixed number of servo sectors (N) with the disks spinning at a fixedrotation speed (w revolutions per second). The sampling rate (orfrequency) f_(s) corresponds to N*w samples per second. The servo burstsin the servo sectors are referred to herein as a stream of positioningbursts, and the bursts of this stream are therefore sensed by a sensingdevice at the rate at which they move past the sensing device, or f_(s).Digital servo controller designs are usually implemented assuming thissampling rate.

Seek acoustic noise generated by a DASD mechanism is a significantproblem. The acoustic noise generated during seek arises from broadbandforcing of the environmental components or from the excitation of “puretone” modes at a well defined frequency f_(s). It is critical tominimize seek acoustics without compromising the access performance of aDASD. Pure tone modes due to structural resonance can get amplifiedthrough the servo feedback process or due to roughness (significant stepchange) in the digital-to-analog converter (“DAC”) output value.

The configuration of a typical DASD servo is shown in FIG. 1. Anexemplary DASD 10 is depicted having a sensing device (actuator 14coupled to read/write electronics 12). It should be understood to thoseskilled in the art that the terms “sensing device” and “access device”used herein should be construed broadly and without limitation toconnote parts of the same subsystem within the DASD, or components ofdifferent subsystems. An access device (actuator 14, voice coil motor(“VCM”) 15, and spindle/VCM Driver 22) is also depicted.

A position error signal (“PES”) 32 (derived from a sensed position burstvia line 30) from peak detecting demodulator 16 is initially sampled byan analog-to-digital converter 26 within a microprocessor unit (“MPU”)18 and sent to the hard disk controller (“HDC”) 20 for PES non-linearityprocessing 24, where known distortion of the PES is first corrected (bythe HDC without burdening the MPU). The linearized PES is sent back tothe MPU for conventional seek servo computation in Seek/Settle/Tk-Followsubsystem 28. During seek, a velocity servo technique is used to producean output positioning control value (referred to herein simply as apositioning value) denoted by U_(n) where n is the sampling instant. Forevery sector denoted by n−1, n, n+1 . . . etc., a new control output orpositioning value is computed (possibly provided by a DAC, and typicallywith some time delay) and sent to the access device via line 34. Thepositioning values 34 are therefore calculated as a function of thesensed positioning (servo) bursts stored on the storage medium.

What is required is a method and system which reduce the noiseassociated with the fixed frequency sampling rates of conventional servosystems, such as the one depicted in FIG. 1. The servo computationcomplexity should be minimized to maintain the use of low cost MPUs inDASD designs.

SUMMARY OF THE INVENTION

The shortcomings of the conventional approaches are overcome by thepresent invention which, in one aspect, is a method and system forpositioning an access device used to access data on a data storagemedium. The data storage medium has a stream of positioning burstsstored thereon. The data storage medium is moved relative to a sensingdevice such that at least some sequential positioning bursts of thestream of positioning bursts move past the sensing device at a firstfrequency. The at least some sequential positioning bursts are sensedwith the sensing device. The access device is positioned using a streamof positioning values generated at a second frequency which is greaterthan the first frequency. One or more of the positioning values of thestream of positioning values are calculated as a function of respectivebursts of the sequential positioning bursts.

For a first positioning burst of the sequential positioning bursts, adesired positioning value is generated relative to a present positioningvalue. An intermediate positioning value is also generated which has anamplitude between the present positioning value and the desiredpositioning value. The generated stream of positioning values (at thesecond frequency) therefore includes the desired positioning value andthe intermediate positioning value. The device is positioned using theintermediate positioning value and the desired positioning value. Theamplitude of the intermediate positioning value may be halfway betweenthe present positioning value and the desired positioning value.

In another aspect, the present invention is a method and system forpositioning an access device used to the access data on the data storagemedium. Again, the data storage medium has a stream of positioningbursts stored thereon. The data storage medium is moved relative to asensing device, and a first positioning burst of the stream ofpositioning bursts is sensed with the sensing device. The access deviceis positioned by asserting at least one positioning value beginning atat least one randomly calculated time point following the sensing of thefirst positioning burst. The at least one positioning value iscalculated as a function of the first positioning burst.

A desired positioning value may be generated relative to a presentpositioning value, and an intermediate positioning value may also begenerated having an amplitude between the present positioning value andthe desired positioning value. In this case, the at least onepositioning value includes the desired positioning value and theintermediate positioning value. The access device is positioned byasserting the intermediate positioning value at a first randomlycalculated time point following the sensing of the first positioningburst, and the access device is then positioned by asserting the desiredpositioning value beginning at a second randomly calculated time.

The sequential positioning bursts may be sensed at a first frequencydetermined by the rate at which the medium is moved past the sensingdevice. In this case, the respective first randomly calculated timepoints and the respective second randomly calculated time points areeach calculated as either a random advance or a random delay fromrespective time points occurring at a second frequency, the secondfrequency being greater than the first frequency.

According to the oversampling, randomized sampling, or randomizedoversampling principles of the present invention, seek acoustic noiseand pure tone build-up are minimized when positioning an access deviceused to access a data storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the following detaileddescription of the preferred embodiment(s) and the accompanying drawingsin which:

FIG. 1 depicts a conventional DASD having a positioning system thereinfor positioning an access device over the data storage medium;

FIG. 2 depicts a stream of output positioning values produced by thepositioning system of FIG. 1;

FIGS. 3a-b depict streams of positioning values provided in accordancewith the present invention;

FIGS. 4a-b depict the streams of FIGS. 3a-b, and the expected resultantpure tone build-ups associated therewith;

FIGS. 5a-d are time and spectral plots of conventional and oversampledoutput positioning value streams;

FIG. 6 is an exemplary DASD modified to produce a positioning valuestream in accordance with the present invention;

FIGS. 7a-d are time-averaged, raster representations of observedcharacteristics of positioning bursts, positioning values, fixedoversampling positioning values, and randomized oversampling positioningvalues, respectively;

FIGS. 8a-b represent a spectral comparison of the waveforms of FIG. 7band FIG. 7c; and

FIGS. 9a-b represent a spectral comparison of the waveforms of FIG. 7band FIG. 7d.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Conventional seek servo systems update the DAC value at a sampling ratecorresponding to the servo-sector rate f_(s), which is determined by therate at which the data storage medium moves past a sensing device. Theservo system output, referred to herein as the positioning value, stepsup or down by an amount computed by the conventional seek servotechnique from U_(n−1) to U_(n) at sampling instant n. Movement of themedium relative to the sensing device can be accomplished by anyrelative type of motion between these two components.

The acoustic noise spectrum produced by the substantial DAC outputchange (U_(n)−U_(n−1)) is reduced and potential for pure tone build upis minimized utilizing the principles of the present invention. Outputroughness and pure tone potential are reduced by computing a new lowroughness DAC output sequence of length “m” which is then oversampled,possibly randomly, at a rate of “mf_(s)” samples per second. Theadditional positioning values generated by the oversampling are referredto herein as intermediate positioning values. U_(n) is referred toherein as the desired positioning value, and U_(n−1) is referred toherein as the present positioning value, with respect to PES_(n).

The simplest output sequence corresponds to the linearly interpolatedvalue(s) between U_(n) and U_(n−1). The resultant finely graduate DACoutput sequence reshapes the acoustic spectrum in a desirable directionwithout compromising the seek performance.

As depicted in FIG. 2, the output curve of a stream of positioningvalues 50 generated by a conventional seek servo undergoes a series ofstep changes for every sector n−2, n−1, n, n+1, etc. Logically, thehigher the DAC step (i.e., the difference between the amplitudes ofU_(n) and U_(n−1)) the larger the undesired acoustic response of theDASD actuator system will be.

As shown in FIG. 3a, and in accordance with the present invention,output amplitude change is minimized, for example, by incrementing thevalue only by 50% of the original magnitude change but in two stepsbased on the doubling of the output sampling rate (i.e., m=2). Ingeneral, the desired amplitude jump can be achieved in m steps within asector time and the output is oversampled at a general frequency ofmf_(s). The conventional output waveform 50 is shown with the overlayed,inventive output waveform 60 in FIG. 3a.

Simplicity of the technique is achieved by maintaining the proven, lowcomplexity control scheme in the MPU intact, and by adding a postoperation in the HDC which is simply a division by an integer number ofthe control step value (e.g., a simple shift operation). No measurablechange in access time is expected by the oversampling operation.However, if warranted, some weighting factors can be used to scale theoversampled sequence, so that the time-integral of the control effort iskept identical to the conventional case at each PES sampling instant. Ina generalized case, the oversampled sequence does not have to be equallyspaced, and an optimum time interval between each sequence can beimplemented specifically for any given DASD hardware structuraldynamics.

The fixed interval oversampling reduces the spectral energy in thefrequency range where the actuator components could be mechanicallyresponsive. However, fixed interval oversampling does not eliminate thepossibility of pure tone generation. A pure tone is generated when theforcing frequency and structural resonance are matched because of afixed time interval DAC output that has a strong phase correlation withpure tone mechanics.

Therefore, in another embodiment of the present invention, the outputtiming of the stream of generated positioning values is randomized. Inother words, rather than generating positioning values at fixedfrequencies f_(s) or mf_(s), the output timing is randomized (e.g., asrandom advances or delays around the otherwise fixed time pointsoccurring at f_(s) or mf_(s)).

By randomizing the DAC output timing, as shown by the output waveform 70in FIG. 3b, the possibility of locking into pure tone forcing isminimized. Both acoustic generation modes (output roughness & pure tone)are minimized by this embodiment of the present invention. In FIG. 3b,it can be observed that the first DAC output can only be delayed (“D”,72 a, 72 b, 72 c) from the fixed sampling time whereas all otheroversampled, intermediate outputs can either be delayed 74 c or advanced(“A”, 74 a, 74 b) with respect to the nominal sampling rate becausetheir magnitude is known a priori.

It should be noted that the present invention involves oversampling ofoutput values, randomizing the times at which output values areprovided, or a combination of these two techniques.

In FIG. 4a, the technique of oversampling is illustrated in which thesystem mechanics consist of a resonant mode with frequency equal to thatof output sampling frequency (2f_(s)) of the servo. A pure tone build-up82 is induced by the fixed sample rate output waveform 60. However, inFIG. 4b, the randomized output sampling rate waveform 70 results in adissipation of the pure tone, as shown by waveform 84.

The results of a “servo-mechanics” model used to evaluate the roughnessminimization oversampling are presented in FIGS. 5a-d. The time domainand spectral plots for conventional (FIGS. 5a-b) and oversampled (m=3)output (FIGS. 5c-d) cases with time domain seek trajectories (102, 112)and actuator currents (100, 110) are shown. In this example it can beseen that the improved actuator current spectrum 116 is generallyreduced in magnitude from the conventional spectrum 106 in the highfrequency range above 1000 Hz.

FIG. 6 depicts an exemplary hardware implementation of a 2.5″ DASD inaccordance with the principles of the present invention. Like referencenumerals are used to designate the like elements shown in FIG. 1.However, unlike FIG. 1, an improved HDC 200 is provided, within whichnow resides unit 204 for computing the intermediate positioning valuesbetween U_(n) (212) and U_(n−1) (206). In this case, the nominal outputsampling was doubled, and thus two positioning values U_(n1) (216) andU_(n2)(214) are calculated as a function of a desired positioning valueU_(n) (212) and present positioning value U_(n−1) (206). The improvedHDC 200 performs the output generation 210 and timing randomization 208operations, in accordance with specific techniques known to thoseskilled in the art. The MPU 18 simply computes the traditional controloutput U_(n) for every PES_(n).

Those skilled in the art will recognize that unit 204 performs thefollowing calculations to produce U_(n1) and Un_(n2):

Δ=U_(n)−U_(n−1)

J=Δ/2

U_(n1)=U_(n−1)+J

U_(n2)=U_(n1)+J

“J” as calculated in this example represents one half of the amplitudejump Δ normally encountered in conventional systems.

FIGS. 7a-7 d show the actual timing (in time-averaged, raster format)used to generate an output data stream, the spectral characteristics ofwhich are depicted in FIGS. 8-9. In the conventional sampling mode, theinterval between two PES samples is 252 us(microseconds) as shown inFIG. 7a. Due to MPU operation and the PES conversion process, there is aminimum delay 300 of 90 us before the first DAC output positioning valueappears. In addition, there is an unavoidable variance of 15 us (minimumto maximum time) present for the conventional positioning values (302 a,302 b) as shown in FIG. 7b.

In the case of fixed frequency oversampling as shown in FIG. 7c, firstDAC output 304 a still occurs after 90 us from receiving the positioning(servo) burst ready signal. The second, intermediate DAC output 306occurs 126 us following the first DAC output, and has no process inducedvariance. The cycle repeats beginning with output 304 b.

For the combination of randomized oversampling, FIG. 7d depicts thetime-averaged intentionally random times at which the positioning valuesare provided. The first DAC output value 308 a has 82 us variance andthe second intermediate DAC output 310 has 50 us variance. Again, thecycle repeats beginning with output value 308 b.

FIG. 8a and FIG. 8b represent a spectral comparison (waveforms 350, 352)of conventional and fixed oversampled voice coil motor (VCM) voltagepower spectra corresponding to the waveforms of FIG. 7b and FIG. 7c. Itcan be seen that the spectrum has been reshaped, and the peak valuepresent at 4 kHz has been eliminated, but a new peak appears at 6 kHzwith much lower (12 dB change) peak value. A second mode at 8 kHz seemsto be unaffected by the oversampling operation.

FIG. 9a and FIG. 9b represent a spectral comparison (waveforms 350, 354)of the conventional and randomized oversampling cases, corresponding tothe waveforms of FIG. 7b and FIG. 7d. The power spectrum for this case(354) appears very close to the fixed oversampled case (352) of FIG. 8b,with further reduction in the 6 kHz peak and a slight increase in the 4kHz peak.

This resultant data confirms that randomized oversampling produces aspectrum comparable to that of fixed oversampling while minimizing thepotential for pure tone acoustic generation. Randomized output istherefore an effective software solution in the presence of samplerate-based “pure tone” mode when a product hardware design or magneticsector format could not be changed readily.

While the invention has been described in detail herein in accordancewith certain preferred embodiments thereof, many modifications andchanges therein may be effected by those skilled in the art.Accordingly, it is intended by the following claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method for positioning an access device used toaccess data on a data storage medium, the data storage medium having astream of positioning bursts stored thereon, the method comprising: (a)moving the data storage medium relative to a sensing device; (b) sensinga first positioning burst of the stream of positioning bursts with thesensing device; and (c) positioning the access device by asserting atleast one positioning value beginning at at least one randomlycalculated time point following the sensing of the first positioningburst, the at least one positioning value being calculated as a functionof the first positioning burst.
 2. The method of claim 1, wherein saidpositioning the access device (c) includes: (i) generating a desiredpositioning value relative to a present positioning value; and (ii)generating an intermediate positioning value having an amplitude betweenthe present positioning value and the desired positioning value, suchthat the at least one positioning value includes the desired positioningvalue and the intermediate positioning value.
 3. The method of claim 2,wherein said positioning the access device (c) further includes: (iii) afirst positioning of the access device by asserting the intermediatepositioning value beginning at a first randomly calculated time point ofthe at least one randomly calculated time point following the sensing ofthe first positioning burst; and (iv) a second positioning of the accessdevice by asserting the desired positioning value beginning at a secondrandomly calculated time point of the at least one randomly calculatedtime point following the sensing of the first positioning burst.
 4. Themethod of claim 3, wherein said sensing (b) includes: sensing at leastsome sequential positioning bursts, including the first positioningburst, of the stream of positioning bursts with the sensing device. 5.The method of claim 4, wherein said positioning the access device (c)includes, for each sequential positioning burst of the at least somesequential positioning bursts sensed with the sensing device: performingsaid generating step (i), said generating step (ii), said firstpositioning step (iii), and said second positioning step (iv).
 6. Themethod of claim 5, wherein the at least some sequential positioningbursts are sensed by said sensing (b) at a first frequency determined bya rate at which the data storage medium is moved past the sensingdevice, and wherein each of at least some of the respective firstrandomly calculated time points and each of at least some of therespective second randomly calculated time points are calculated aseither a random advance or a random delay from respective time pointsoccurring at a second frequency, the second frequency being greater thanthe first frequency.
 7. The method of claim 6, wherein the secondfrequency is an integer multiple of the first frequency.
 8. The methodof claim 7, wherein the second frequency is two or three times the firstfrequency.
 9. The method of claim 1, wherein the sensing device and theaccess device are the same device, or components of the same actuator orhead system.
 10. A method for positioning an access device used toaccess data on a data storage medium, the data storage medium having astream of positioning bursts stored thereon, the method comprising:moving the data storage medium relative to a sensing device; sensing afirst positioning burst of the stream of positioning bursts with thesensing device; generating at least two positioning signal values as afunction of the first positioning burst and a desired position for theaccess device; and positioning the access device at the desired positionby asserting the at least two positioning values at at least tworespective times; wherein the at least two positioning values include atleast one intermediate positioning value and a desired positioningvalue, each of the at least one intermediate positioning value having avalue between the desired positioning value and a present positioningvalue, the desired positioning value corresponding to the desiredposition of the access device; and wherein the at least one intermediatepositioning value comprises a single intermediate positioning value, andwherein the two respective times at which the single intermediatepositioning value and the desired positioning value are asserted arerandomly calculated.
 11. The method of claim 10, wherein the tworandomly calculated times at which the single intermediate positioningvalue and the desired positioning value are asserted each comprises arandom advance or a random delay from respective time points separatedby one half of the interval between which the first positioning burstand a second sequential positioning burst of the stream of positioningbursts are sensed by the sensing device.
 12. A method for positioning anaccess device used to access data on a data storage medium, the datastorage medium having a stream of positioning bursts stored thereon, themethod comprising: moving the data storage medium relative to a sensingdevice; sensing a first positioning burst of the stream of positioningbursts with the sensing device; generating at least two positioningsignal values as a function of the first positioning burst and a desiredposition for the access device; and positioning the access device at thedesired position by asserting the at least two positioning values at atleast two respective times; wherein said sensing includes: sensing atleast some sequential positioning bursts, including the firstpositioning burst, of the stream of positioning bursts with the sensingdevice; and wherein the at least some sequential bursts are sensed at afirst frequency determined by a rate at which the data storage medium ismoved past the sensing device, and wherein at least some of the at leasttwo respective times are each calculated as either a random advance or arandom delay from time points occurring at a second frequency, thesecond frequency being greater than the first frequency.
 13. The methodof claim 12, wherein the second frequency is an integer multiple of thefirst frequency.
 14. The method of claim 13, wherein the secondfrequency is two or three times the first frequency.
 15. A system forpositioning an access device used to access data on a data storagemedium, the data storage medium having a stream of positioning burstsstored thereon, the system comprising: (a) means for moving the datastorage medium relative to a sensing device; (b) means for sensing afirst positioning burst of the stream of positioning bursts with thesensing device; and (c) means for positioning the access device byasserting at least one positioning value beginning at at least onerandomly calculated time point following the sensing of the firstpositioning burst, the at least one positioning value being calculatedas a function of the first positioning burst.
 16. The system of claim15, wherein said means for positioning the access device (c) includes:(i) means for generating a desired positioning value relative to apresent positioning value; and (ii) means for generating an intermediatepositioning value having an amplitude between the present positioningvalue and the desired positioning value, such that the at least onepositioning value includes the desired positioning value and theintermediate positioning value.
 17. The system of claim 16, wherein saidmeans for positioning the access device (c) further includes: (iii)means for a first positioning of the access device by asserting theintermediate positioning value beginning at a first randomly calculatedtime point of the at least one randomly calculated time point followingthe sensing of the first positioning burst; and (iv) means for a secondpositioning of the access device by asserting the desired positioningvalue beginning at a second randomly calculated time point of the atleast one randomly calculated time point following the sensing of thefirst positioning burst.
 18. The system of claim 17, wherein said meansfor sensing (b): senses at least some sequential positioning bursts,including the first positioning burst, of the stream of positioningbursts with the sensing device.
 19. The system of claim 18, wherein saidmeans for positioning the access device (c) performs, for eachsequential positioning burst of the at least some sequential positioningbursts sensed with the sensing device: said generating (i), saidgenerating (ii), said first positioning (iii), and said secondpositioning (iv).
 20. The system of claim 19, wherein the at least somesequential positioning bursts are sensed by said means for sensing (b)at a first frequency determined by a rate at which the data storagemedium is moved past the sensing device, and wherein each of at leastsome of the respective first randomly calculated time points and each ofat least some of the respective second randomly calculated time pointsare calculated as either a random advance or a random delay fromrespective time points occurring at a second frequency, the secondfrequency being greater than the first frequency.
 21. The system ofclaim 20, wherein the second frequency is an integer multiple of thefirst frequency.
 22. The system of claim 21, wherein the secondfrequency is two or three times the first frequency.
 23. The system ofclaim 15, wherein the sensing device and the access device are the samedevice, or components of the same actuator or head system.
 24. A systemfor positioning an access device used to access data on a data storagemedium the data storage medium having a stream of positioning burstsstored thereon, the system comprising: means for moving the data storagemedium relative to a sensing device; means for sensing a firstpositioning burst of the stream of positioning bursts with the sensingdevice; means for generating at least two positioning signal values as afunction of the first positioning burst and a desired position for theaccess device; and means for positioning the access device at thedesired position by asserting the at least two positioning values at atleast two respective times; wherein the at least two positioning valuesinclude at least one intermediate positioning value and a desiredpositioning value, each of the at least one intermediate positioningvalue having a value between the desired positioning value and a presentpositioning value, the desired positioning value corresponding to thedesired position of the access device; and wherein the at least oneintermediate positioning value comprises a single intermediatepositioning value, and wherein the two respective times at which thesingle intermediate positioning value and the desired positioning valueare asserted are randomly calculated.
 25. The system of claim 24,wherein the two randomly calculated times at which the singleintermediate positioning value and the desired positioning value areasserted each comprises a random advance or a random delay fromrespective time points separated by one half of the interval betweenwhich the first positioning burst and a second sequential positioningburst of the stream of positioning bursts are sensed by the sensingdevice.
 26. A system for positioning an access device used to accessdata on a data storage medium, the data storage medium having a streamof positioning bursts stored thereon, the system comprising: means formoving the data storage medium relative to a sensing device; means forsensing a first positioning burst of the stream of positioning burstswith the sensing device; means for generating at least two positioningsignal values as a function of the first positioning burst and a desiredposition for the access device; and means for positioning the accessdevice at the desired position by asserting the at least two positioningvalues at at least two respective times; wherein said means for sensing:senses at least some sequential positioning bursts, including the firstpositioning burst, of the stream of positioning bursts with the sensingdevice; and wherein the at least some sequential bursts are sensed at afirst frequency determined by a rate at which the data storage medium ismoved past the sensing device, and wherein at least some of the at leasttwo respective times are each calculated as either a random advance or arandom delay from time points occurring at a second frequency, thesecond frequency being greater than the first frequency.
 27. The systemof claim 26, wherein the second frequency is an integer multiple of thefirst frequency.
 28. The system of claim 27, wherein the secondfrequency is two or three times the first frequency.